The voltage-sensitive sodium channel in Drosophila melanogaster is being studied using two neurotoxins, tetrodotoxin and saxitoxin, which are both known to bind specifically to sodium channels and block the action potential. A rapid filtration assay has been developed to study 3H-saxitoxin binding to extracts of Drosophila heads. Initial studies have shown that the toxin binds to Drosophila extracts with high affinity. This binding is saturable, reversible, and noncooperative, and is inhibited by low concentrations of tetrodotoxin. Saxitoxin binding will be studied in mutants which are abnormally sensitive to tetrodotoxin and in mutants which exhibit temperature-induced defects in propagation of action potentials to determine if any of these mutants are altered in the saxitoxin-binding component of the sodium channel. Specifically, we will test for alterations in the number of binding sites as well as for alterations in other parameters including KD, offrate, and KI for inhibition by tetrodotoxin. In addition, for the temperature-sensitive mutants we will study the thermal inactivation of the binding activity and will define the effects of pH and temperature on the binding reaction itself. These studies provide a model system for investigating the organization of genes coding for products involved in cell excitability.